Presently, long-term suppression of viral replication with antiretroviral drugs is the only option for treating HIV-1 infection. To date, a number of approved drugs have been shown to greatly increase patient survival. However, therapeutic regimens known as highly active antiretroviral therapy (HAART) are often complex because a combination of different drugs must be administered to the patient to avoid the rapid emergence of drug-resistant HIV-1 variants. Despite the positive impact of HAART on patient survival, drug resistance can still occur.
The emergence of multidrug-resistant (MDR) HIV-1 isolates has serious clinical consequences and must be suppressed with a new drug regimen, known as salvage therapy. Current guidelines recommend that salvage therapy includes at least two, and preferably three, fully active drugs. Typically, first-line therapies combine three to four drugs targeting the viral enzymes RT and protease (PR). One option for salvage therapy is to administer different combinations of drugs from the same mechanistic class that remain active against the resistant isolates. However, the options for this approach are often limited, as resistant mutations frequently confer broad cross-resistance to different drugs in the same class. Alternative therapeutic strategies have recently become available with the development of fusion, entry, and integrase (IN) inhibitors. However, resistance to all three new drug classes has already been reported both in vitro and in vivo. Sustained successful treatment of HIV-1-infected patients with antiretroviral drugs will therefore require the continued development of new and improved drugs with new targets and mechanisms of action.
The HIV Gag polyprotein precursor (Pr55Gag), which is composed of four protein domains—matrix (MA), capsid (CA), nucleocapsid (NC) and p6—and two spacer peptides, SP1 and SP2, represents a new therapeutic target. Although the cleavage of the Gag polyprotein plays a central role in the progression of infectious virus particle production, to date, no antiretroviral drug has been approved for this mechanism.
In most cell types, assembly occurs at the plasma membrane, and the MA domain of Gag mediates membrane binding. Assembly is completed by budding of the immature particle from the cell. Concomitant with particle release, the virally encoded PR cleaves Gag into the four mature protein domains, MA, CA, NC and p6, and the two spacer peptides, SP1 and SP2. Gag-Pol is also cleaved by PR, liberating the viral enzymes PR, RT and IN. Gag proteolytic processing induces a morphological rearrangement within the particle, known as maturation. Maturation converts the immature, donut-shaped particle to the mature virion, which contains a condensed conical core composed of a CA shell surrounding the viral RNA genome in a complex with NC and the viral enzymes RT and IN. Maturation prepares the virus for infection of a new cell and is absolutely essential for particle infectivity.
Bevirimat (PA-457) is a maturation inhibitor that inhibits the final step in the processing of Gag, the conversion of capsid-SP1 (p25) to capsid, which is required for the formation of infectious viral particles. Bevirimat has activity against ART-resistant and wild-type HIV, and has shown synergy with antiretrovirals from all classes. Bevirimat reduced HIV viral load by a mean of 1.3 log10/mL in patients who achieved trough levels of >=20 μg/mL and who did not have any of the key baseline Gag polymorphisms at Q369, V370 or T371. However, Bevirimat users with Gag polymorphisms at Q369, V370 or T371 demonstrated significantly lower load reductions than patients without Gag polymorphisms at these sites.
It would therefore be an advance in the art to discover alternative compounds that are effective for the prevention and/or treatment of HIV infections.